This application claims the priority of German application 198 17 320.2, filed in Germany on Apr. 18, 1998, the disclosure of which is expressly incorporated by reference herein.
The invention relates to an injector for fuel injection systems of the type comprising
an injector housing in which a piezoelectric stack is located, PA1 a valve housing connected with the injector housing in which a valve closing device, which can be operated by the piezoelectric stack, and provided with a jet needle, is displaceably mounted, PA1 a return device being provided by means of which the valve closing device can be returned, PA1 a displacement piston actuated by the piezoelectric stack being located between the piezoelectric stack and the jet needle of the valve closing device, and PA1 a control piston located downstream from the displacement piston that increases the adjustment travel.
An injector of the above noted general type is known from German Patent Document DE 195 19 191 C2. A hydraulic distance transformation unit is located between a piezoelectric stack and the jet needle of the injector. This unit has a displacement piston and a control piston located downstream from the displacement piston. However, the fact that the actuating force for the jet needle decreases during the travel transformation is disadvantageous.
A fuel injector for internal combustion engines is known from German Patent Document DE 195 00 706 A1, said valve having a hydraulic travel amplifier for converting a travel of the piezoelectric actuator. In this valve, passages that supply a fluid and carry fluid away are separate from one another, with the fluid being guided into an annular space by a passage located in the valve housing. However, the disadvantage of this injector is that, although the travel is amplified, the actuating force is reduced at the same time by the law of the lever. It is also disadvantageous that the passage of the fuel injector is subjected to a bending stress while fuel is being supplied to the annular chamber.
Reference is made regarding additional prior art to European Patent Document EP 0 218 895 B1, from which a metering valve for metering fluids or gases with a piezoelectric actuator is known. The pressure with which the valve is actuated acts on the piezoelectric actuator directly. At the pressures of approximately 1000 bars that develop in fuel injection systems, exact function of the valve is no longer guaranteed because of losses in the actuating travel of the jet needle. It is also disadvantageous that, after the jet needle lifts out of the valve seat, the fuel sprays uncontrollably into the combustion chamber through the resulting gap.
A goal of the present invention is to provide an injector of the type referred to above with which fuel injection can be performed with high accuracy and precision and without loss of fuel by transformation of the travel.
According to the invention, this goal is achieved by providing an arrangement wherein a working piston is provided for hydraulic following amplification that actuates the jet needle and increases the actuating force.
By using a hydraulic follower amplifier in the form of a working piston it is possible to decouple the system in terms of force. The travel of the piezoelectric stack is transmitted to a displacement piston. A control piston connected downstream from the displacement piston which increases the adjustment travel produced by the piezoelectric stack moves at a specified transformation ratio toward the jet needle. The jet needle is then actuated by a working piston that increases the actuating force.
The travel amplification according to the invention is decoupled from the force because the application of force to open the jet needle comes only from the system pressure, for example a rail pressure. Since there is no loss of power in the transformation, the actuation of the piezoelectric stack also does not have a negative influence on the opening of the jet needle.
In a highly advantageous improvement of certain preferred embodiments of the invention, provision is made such that a pressure compensating chamber is located for a hydraulic length compensation of the piezoelectric stack between the displacement piston and the control piston, said chamber being connected on one side with an overflow line of the control piston and on the other side with an overflow line of the displacement piston.
The pressure compensating chamber according to the invention together with its hydraulic compensating volume serves to compensate temperature and elongation effects of the piezoelectric stack.
In another likewise highly advantageous feature of certain preferred embodiments of the invention, provision can also be made for a pressure pad to be located between the jet needle and the working piston for hydraulic length compensation for the jet needle, with a length compensating chamber with a compensating spring being located between the pressure pad and the working piston.
As a result of this design according to the invention, hydraulic length compensation is achieved for the jet needle, due to thermal and hydraulic changes in length.
The injector according to the invention is suitable for jet needles that open outward as well as those that open inward using the same operating principle.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.